Recent years have seen increasing demand for miniaturization and increased reliability of dielectric elements as electronic circuits reach higher densities. Miniaturization of electronic components such as laminated ceramic capacitors, together with increased capacity and higher reliability are rapidly progressing, while the applications of electronic components such as laminated ceramic capacitors are also expanding. Various characteristics are required as these applications expand.
For example, there is a growing shift from silicon to silicon carbide in semiconductors used for circuits in AC-DC inverters and DC-DC converters for motor vehicles. There is a need for even greater reliability in capacitors used around silicon carbide semiconductors. Specifically, there is a need for a high dielectric constant when a high DC bias is applied. Furthermore, there is also a need for a long high-temperature load lifespan in order to increase the lifespan when a high voltage is applied under a high temperature. In addition, there is simultaneously a need for high mechanical strength in order to prevent cracking and splintering etc. during production of the dielectric material and mounting on a substrate.
In order to respond to these requirements, dielectric compositions having barium titanate (BaTiO3) as the main component are often investigated and used in general. However, there are problems with electronic components which have dielectric layers comprising a conventional dielectric composition having BaTiO3 as the main component in that the dielectric constant decreases when a high DC bias is applied. It is difficult to avoid this problem because BaTiO3 is a ferroelectric material. In addition, the higher the DC bias, the more the dielectric constant tends to decrease. When such electronic components are used for applications involving high DC bias application it is therefore necessary to anticipate the amount of reduction in the dielectric constant and to use a plurality of the electronic components connected in parallel in order to maintain the required capacitance or dielectric constant. Methods for connecting a plurality of the electronic components in parallel are a particular problem in terms of high cost.
Furthermore, a laminated ceramic capacitor which has dielectric layers comprising a conventional dielectric composition having BaTiO3 as the main component has a relatively good high-temperature load lifespan. However, it is anticipated that the environments under which electronic components are used will become even more harsh in the future so a further improvement in the high-temperature load lifespan would be desirable.
When a conventional dielectric composition having BaTiO3 as the main component is used for applications under a low DC bias of several volts or less, the field intensity applied to the dielectric layers is small, so the thickness of the dielectric layers can be set to a sufficiently thin level that breakdown does not occur. It is therefore possible to reduce the size of the dielectric element. In addition, there are also very few cases in which cracking etc. becomes a problem because of external stress or the like to which the electronic component is subjected during production of the dielectric material and mounting on a substrate. However, for applications involving usage under a high DC bias of several hundred volts or greater, the dielectric layers must be sufficiently thick to ensure safety. The dielectric material therefore tends to be larger and heavier for applications involving usage under a high DC bias. The mechanical strength required also increases as a result. The dielectric material may crack or splinter if it is dropped during production because it is not possible to ensure adequate mechanical strength which is commensurate with the size and weight of the dielectric material.
In order to solve these problems, Japanese Patent Application JP 2006 206362 describes a dielectric porcelain having barium titanate as the main component and containing Ca, Sr, Mg, Mn and rare earth elements, and characterized by a core-shell structure in which the Ca concentration is greater at the particle surface than at the centre of the particle, and the Sr, Mg, Mn and rare earth elements are unevenly distributed at the particle surface.
Furthermore, Japanese Patent Application JP 2005 22891 describes a dielectric porcelain comprising both perovskite barium titanate crystal grains in which part of the B site is substituted with Zr (BTZ-type crystal grains), and perovskite bismuth sodium titanate crystal grains in which part of the A-site is substituted with Sr (BNST-type crystal grains). That dielectric porcelain is characterized by a core-shell structure in which Mg, Mn and at least one rare earth element are present in the grain boundary phase between the BTZ-type crystal grains and the BNST-type crystal grains, and the mean particle size of both the BTZ-type crystal grains and the BNST-type crystal grains is 0.3-1.0 μm.
Dielectric porcelain comprising BaTiO3 as the main component and having a core-shell structure such as that described in Japanese Patent Application JP 2006 206362 has a relatively high dielectric constant value of 2500 or greater at 20° C. when a DC bias is not applied. However, a sufficiently good value is not exhibited for the rate of change in the dielectric constant or the rate of change in capacitance when a DC bias of 5 V/μm is applied. Furthermore, a sufficiently good value is not exhibited for the high-temperature load lifespan. In addition, there is no mention of the mechanical strength.
On the other hand, the ceramic composition described in Japanese Patent Application JP 2005 22891 has a relatively high dielectric constant when a DC bias is not applied, and the DC bias characteristics when a DC bias of 3 V/μm is applied are also less than −20%. However, the value cannot be considered sufficient for use under a high voltage, such as in a DC-DC converter or AC-DC inverter for a motor vehicle. Furthermore, there is no mention of the high-temperature load lifespan or mechanical strength.